#include<vector>
using namespace std;

struct TreeNode {
     int val;
    TreeNode *left;
     TreeNode *right;
     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 };

 class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        //中止条件
        if(root == NULL){
            return NULL;
        }
        //中止条件
        if(root == p || root == q){
            return root;
        }

        TreeNode* left = lowestCommonAncestor(root->left, p, q);
        TreeNode *right = lowestCommonAncestor(root->right, p, q);

        if(left!=NULL && right!=NULL){
            return root;
        }

        if(left!=NULL){
            return left;
        }

        if(right!=NULL){
            return right;
        }

        return NULL;
    }
};